1. Field of the Invention
The present invention provides a method for separating halocarbons. In particular, the invention provides a method for separating 2-chloro-1,1,1,2-tetrafluoropropane (HCFC-244bb) from 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf) based on differences in freezing points of these compounds. More particularly the invention pertains to a method for separating HCFC-244bb from HCFO-1233xf, which are useful as intermediates in the production of 2,3,3,3-tetrafluoropropene (HFO-1234yf).
2. Description of the Related Art
Fluorocarbon based fluids have found widespread use in industry in a number of applications, including as refrigerants, aerosol propellants, blowing agents, heat transfer media, and gaseous dielectrics. Because of the suspected environmental problems associated with the use of some of these fluids, including the relatively high global warming potentials associated therewith, it is desirable to use fluids having the lowest possible greenhouse warming potential in addition to zero ozone depletion potential. Thus there is considerable interest in developing environmentally friendlier materials for the applications mentioned above. Tetrafluoropropenes, having zero ozone depletion and low global warming potential, have been identified as potentially filling this need. However, the toxicity, boiling point, and other physical properties in this class of chemicals vary greatly from isomer to isomer. One tetrafluoropropene having valuable properties is 2,3,3,3-tetrafluoropropene (HFO-1234yf). Thus, there is a need for new manufacturing processes for the production of tetrafluoropropenes and in particular 2,3,3,3-tetrafluoropropene.
HCFC-244bb and HCFO-1233xf are intermediates in the production of 2,3,3,3-tetrafluoropropene (HFO-1234yf) which is well known in the art as described in U.S. Applications 20070007488 and 20070197842, the specifications of which are incorporated herein by reference. HFO-1234yf has been disclosed to be an effective refrigerant, heat transfer medium, propellant, foaming agent, blowing agent, gaseous dielectric, sterilant carrier, polymerization medium, particulate removal fluid, carrier fluid, buffing abrasive agent, displacement drying agent and power cycle working fluid.
Mixtures of two halocarbons are often inseparable using standard techniques, especially when then form a binary azeotrope or azeotrope-like composition. The prior art has suggested various methods of separating azeotropic mixtures of fluorocarbons. In this regard European patent application EP 0 472 391 suggests separating HFC-134a from a mixture containing hydrochlorofluorocarbons using an extraction agent such as trichloroethylene or perchloroethylene, among others. U.S. Pat. No. 5,211,817 attempts a separation of fluorocarbons from azeotropic mixtures with HF by column distillation and withdrawing a vapor side-stream followed by introducing the side-stream into a rectifying column equipped with a condenser and operated at a high reflux ratio. These provide less than satisfactory solutions to the problem.
It has now be found that individual halocarbons can be independently separated from a composition of two different halocarbons by freezing the composition at a temperature at or below the freezing point of the first halocarbon but above the freezing point of the second halocarbon.
HCFC-244bb and HCFO-1233xf are inseparable using conventional separation techniques known in the art since HCFC-244bb and HCFO-1233xf form a binary azeotrope or azeotrope-like composition which is described in U.S. Provisional Application 61/040,759 filed Mar. 31, 2008, the specification of which is incorporated herein by reference. It has been found that HCFC-244bb freezes at a temperature of about −78° C. while HCFO-1233xf does not freeze at this temperature. When it is desired to separate HCFC-244bb from HCFO-1233xf, the mixture of HCFC-244bb and HCFO-1233xf can be cooled to a temperature below the freezing point of HCFC-244bb but above the freezing point of HCFO-1233xf and then the two compounds can be separated by removing liquid or gaseous HCFO-1233xf from solid HCFC-244bb by decantation, filtration, use of centrifuge, or other means known in the art. Essentially pure HCFC-244bb and HCFO-1233xf can be recovered.